In systems for manufacturing semiconductor components, the spatial position of certain components that are displaceable relative to one another are determined with precision with the aid of suitable position-measuring devices. Using the determined positional data, a computer-controlled sequencing control can then be implemented in these systems. Conventionally, the requisite position measuring is predominantly performed via a plurality of laser interferometers. In the future, it is assumed that the accuracy requirements for the position measurement will continue to increase with simultaneously increasing positioning speeds of the various parts. Given the high accuracy requirements that will then result, laser interferometers may no longer be able to be used as position-measuring devices. Even with optimal air conditioning, the refractive index fluctuations in the surrounding air may lead to unacceptable measuring value fluctuations in the position determination, which are on the order of magnitude of a few nanometers (nm).
For this reason, alternative position-measuring devices have already been proposed for such systems. European Published Patent Application No. 1 019 669, for example, describes the use of optical position-measuring devices having so-called cross gratings as a two-dimensional measuring graduation. In the following text, such position-measuring devices will also be referred to as grating-based position-measuring devices. These systems are hardly affected by possible refractive index fluctuations of the air and therefore allow position measurements that are well reproducible.
Optical position-measuring devices having gratings as measuring graduations, which provide the necessary resolutions in the nanometer range, are usually based on interferential scanning principles. In this instance, in general, a light beam from a suitable light source is split up into a least two coherent partial beams of rays, which subsequently impinge upon a plurality of gratings in the scanning beam path before they are reunited and brought to interference. The position information that is ultimately of interest is provided by the (displacement-dependent) phase position of the two interfering partial beams of rays. The resulting path length difference is usually approaching zero for the two partial beams of rays between splitting up and reuniting in symmetrical scanning beam path variants of such systems. Therefore, a short coherence length of the light used is sufficient to provide the desired interference from the detection side.
German Published Patent Application No. 10 2005 043 569 describes an additional interferential position-measuring device, which has asymmetric scanning beam paths for the two partial beams of rays. This means that, based on this asymmetry, path length differences result for the partial beams of rays coming to interference, on the order of magnitude up to a few millimeters (mm). With regard to the required coherence length of the light used, this means that the coherence length has to be in the range of a few millimeters (mm) all the way up to a few centimeters (cm), otherwise no interference of the partial beams of rays that come to superposition is possible.